Within the context of the prediction, detection and attribution of climate change, a number of studies have explicitly or implicitly assumed that individual climate responses to individual forcing agents can be linearly added to obtain the total climate response to the sum of the forcing agents. This assumption of the “linear additivity of forcing-response relationships” has been tested by previous studies, but it remains controversial whether linear additivity holds with all combinations of forcing agents, such as “greenhouse gases plus indirect effects of anthropogenic aerosols” or “greenhouse gases plus solar irradiance.” This study explored whether linear additivity holds in 5-yr mean temperature/precipitation responses to various combinations of forcing agents in the 20th century and in a future-emissions scenario at global and continental scales. We used Model for Interdisciplinary Research on Climate version 3 (MIROC3), which includes the 1st and 2nd indirect effects of aerosols. The forcing factors considered were well-mixed greenhouse gases, the direct and indirect effects of sulphate and carbon aerosols, ozone, land-use changes, solar irradiance and volcanic aerosols (the latter three factors were specified only in the 20th-century runs). By performing and analysing an enormous matrix of forcing runs, we concluded that linear additivity holds in temperature responses for all of the combinations of forcing agents at the global and continental scales, but it breaks down for precipitation responses in certain cases of future projections.
Given the importance and difficulty in evaluating long-term trends in the tropical low-cloud amount (Cl), we examined mechanisms that determine the Cl trend in 20th century experiments using two different versions of the climate model called the Model for Interdisciplinary Research on Climate (MIROC). The Cl trend patterns are coherent with trends in vertical velocity (ω) and lower-tropospheric stability (LTS). While the mean LTS trend varies and gives a stronger constraint to the Cl trends, the ω trend cannot do so due to mass conservation. Two of three reanalysis products support the positive LTS trend, but it is inconclusive because of the diversity in pattern and sign. Copyright © 2011 Royal Meteorological Society
Detection/attribution analyses of temperatureextremes were carried out by comparing a new griddedobservational dataset of daily maximum/minimumtemperatures (HadGHCND) and the simulation ofMIROC3.2. It was shown that significant anthropogenicwarming is detectable in the annual warmest night, andthe coldest day and night from 1950 to 1999, whilehuman influence was not detected in the warmest day.These findings are in agreement with a previous studythat examined the simulation of HadCM3. Human influenceis also identified in the decrease in the number offrost days, but not with the increase in the number ofsummer days. Furthermore, it was suggested that halfof the warming trend due to rising greenhouse gas concentrationsis canceled out by other factors, predominantlyaerosol cooling. It is expected that a rapid declineof aerosol emissions coupled with rising greenhouse gasconcentrations would induce larger changes in temperatureextremes in the future.